Typically, various movement mechanisms are used in the machine tool in order to move a workpiece (an object to be machined) and a tool for machining the workpiece to any relative positions.
For instance, linear movement mechanisms are respectively provided in X axis, Y axis and Z axis to a support structure of a table on which the workpiece is placed or a support structure of a head to which the tool is attached in order to move the workpiece and/or the tool in three dimensions. Moreover, a rotary movement mechanism is used for changing a posture of the table and/or the head.
Each of the movement mechanisms includes: two relatively movable members (e.g., a guide member and a movement member movable along the guide member); a drive mechanism for moving the two members; and a guide mechanism for securing accuracy (guiding accuracy) of a moving direction or a movement axis.
Such a guide mechanism is required to have a high guiding accuracy, in other words, a geometrical accuracy showing that a linear movement is conducted in a line as straight as possible and a rotational movement is conducted in a circle as perfect as possible. Further, the guide mechanism is required to have a high load capacity, a low friction and a high damping performance (vibration absorption performance)
Recently, a hydrostatic pressure guide mechanism is used in the guide mechanism for the machine tool (Patent Literature 1: JP-A-2004-58192).
In the hydrostatic pressure guide mechanism, a static pressure chamber is formed on one of a pair of slide surfaces. A lubricating oil is supplied into the static pressure chamber, whereby a load is transmitted by the static pressure to the other of the slide surfaces. In other words, only the lubricating oil intervenes between the pair of slide surfaces, so that the pair of slide surfaces are in non-contact with each other, thereby significantly reducing the slide resistance.
On the other hand, a traditional sliding guide mechanism (dynamic pressure guide mechanism) is often used as the guide mechanism for the machine tool (Patent Literature 2: JP-A-2008-238397).
The sliding guide mechanism is configured to slide a pair of smooth slide surfaces while supplying a lubricating oil therebetween. The pair of slide surfaces are kept in solid contact with each other while being lubricated with a lubricating oil.
Since an oil film, irrespective of the still or moving oil film, constantly intervenes in the above-described hydrostatic pressure guide mechanism, a high load can be supported and a low friction can be stably achieved.
However, since the hydrostatic pressure guide mechanism is configured to float an object using the oil film, a damping performance of the hydrostatic pressure guide mechanism is limitative. Moreover, the hydrostatic pressure guide mechanism requires a supply device for supplying the lubricating oil for forming the oil film and a recovery device for recovering the lubricating oil. Especially, a typical hydrostatic pressure guide mechanism using the lubricating oil cannot discharge the lubricating oil to the outside unlike an air static pressure bearing using air. Accordingly, the lubricating oil supplied to a static pressure chamber is discharged from an outer circumferential edge to the outside of the guide mechanism. Particularly, since a huge amount of the lubricating oil is discharged in the hydrostatic pressure guide mechanism as compared with the sliding guide, the recovery device for recovering the lubricating oil and returning the lubricating oil to the supply device is required. Accordingly, arrangements of devices and pipes associated with the guide mechanism are complicated.
On the other hand, since the sliding guide mechanism provides a sliding guide between the pair of slide surfaces, a guiding accuracy and the damping performance can be improved and the structure is simple. However, in the sliding guide mechanism, since a load capacity is small and a friction coefficient is large, particularly, a friction coefficient is increased when the sliding guide mechanism is started and/or driven at a low speed, motion occasionally becomes unsmooth to affect a positioning accuracy.
In order to smooth the motion of the machine tool, it is conceivable to replace a typical sliding guide mechanism with a hydrostatic pressure guide mechanism excellent in a low friction performance as the guide mechanism.
However, even if a typical sliding guide mechanism is simply replaced by the hydrostatic pressure guide mechanism, a desired performance may fail to be obtained because of the above-described difference in characteristics.
Alternatively, it is conceivable to use a typical sliding guide mechanism and the hydrostatic pressure guide mechanism in combination.
However, a typical hydrostatic pressure guide mechanism is configured such that the lubricating oil supplied to a static pressure chamber is discharged from an outer circumferential edge to the outside of a slide structure.
Accordingly, when the sliding guide mechanism and the hydrostatic pressure guide mechanism are used in combination, the lubricating oil discharged to the outside is likely to be unrecovered and overflow. The overflowing lubricating oil is likely to reach the sliding guide mechanism to adversely influence the sliding guide mechanism.